Lapping is a well established process for finishing the tooth surfaces of bevel gears. It is a process that provides an economical alternative to other hard finishing processes for bevel gears and it has been used in all areas except for some aircraft applications.
In the lapping process, a pinion and ring gear are mounted, via appropriate workholding equipment, to respective spindles in a lapping machine, which has the same basic design as a testing machine. The spindles may be of the direct drive type of may be driven by a motor, gear train, or belt and pulley system as is known to the skilled artisan. In most instances of rolling of the gearset, the pinion is the driving member and the ring gear is braked. The gears are rolled in mesh and lapping compound, which can be a mixture of oil (or water) and silicon carbide or similar abrasive, is poured into the meshing zone. Examples of lapping and/or testing machines can be found in U.S. Pat. No. 3,099,901 to Hunkeler; U.S. Pat. No. 3,142,940 to Rebeski; U.S. Pat. No. 3,717,958 and U.S. Pat. No. 3,807,094 to Ellwanger et al.; U.S. Pat. No. 5,609,058 to Gnadt et al., and U.S. Pat. No. 6,120,355 to Stadtfeld et al.
Most lapping and testing machines have three degrees of freedom available for realizing relative motion between a ring gear and pinion. The first freedom being relative movement in the direction of the ring gear axis which shall be referred to as direction G or the G axis, the second freedom being relative movement in direction of the pinion axis which shall be referred to as direction H or the H axis, and the third degree of freedom being distance between the ring gear and pinion axes which shall be referred to as direction V or the V axis. The direction V is also known as the “hypoid offset.”
In lapping or testing processes, relative movement in the V and H directions effect positional changes in the contact pattern of the members of the gearset, in effect modifying the contact pattern. Lapping involves rotating the gear members in mesh and under a load with contact at a desired position on the tooth surfaces. Thus, the members are located at particular V and H positions along with a particular G axis position to effect the desired backlash. As the gear set is lapped, contact is shifted toward one of the outer (heel) or inner (toe) portions of the tooth surface by changing the V and H settings as necessary to effect such a shifting of the contact position. As V and H are changed to effect the shifting, the G axis position must also be changed to maintain the desired backlash.
Lapping operations may be carried out at pinion spindle speeds ranging from about 1000 RPM to about 4000 RPM. At the higher speeds (for example 2500-4000 RPM), imperfections in spindle construction, drive train construction, gear set motion transmission error, or running conditions, although slight, can be magnified to produce undesirable periodic changes in driving moments which can build to amplitude levels such that the tooth bearing patterns in the finished product become difficult to control by the lapping process.
One solution to the above problem is addressed in U.S. Pat. No. 3,807,094 to Ellwanger et al. wherein a lapping machine comprises an elastomeric coupling mounted between a pair of mounting blocks on the spindle associated with a braking means (usually the driven spindle to which a gear member is mounted). The elastomeric coupling twists, thus allowing one mounting block to rotate relative to the other mounting block, to isolate and absorb any aberrations in rotational moments due to braking of the spindle or other imperfections as discussed above.
The elastomeric coupling discussed above exhibits a fixed combination of damping and spring characteristics which are not necessarily well matched to the spindle and gear inertia. Therefore, where undesirable effects are best isolated and absorbed by controlling the total system dynamics in terms of inertia, damping and spring rate, an elastomeric coupling may provide little relief of the problem.
European Patent Application No. 0061086 discloses a spindle insert supported by an elastic element that adjusts to vibrations caused by the meshing action of gear members mounted on the machine spindles because of the lower mass moment of inertia of the spindle insert. The disclosed system is a passive system reacting only to vibrations of meshed gears on the machine.
U.S. Pat. No. 6,481,508 to McGlasson et al. teaches a machine tool spindle comprising a rear spindle portion and a forward spindle portion, with one or more spring elements extending between the rear spindle portion and the forward spindle portion. The relative rotational position of the rear and forward spindle portions is controlled during machine operation to effect a predetermined deflection of the spring elements thereby resulting in a predetermined winding-up of the spring elements and the creation of a desired and controllable amount of torque between members of a meshed gearset comprising a first gearset member mounted on the first spindle and a second gearset member mounted on the second spindle.
It is known for machines to employ a mechanical brake on the gear spindle, or electronic torque control on the gear spindle motor, to control the load applied between the teeth of the gearset during testing or lapping. However, these methods have not led to good control of the dynamic load variations occurring as a function of gear or pinion rotation and tooth-to-tooth rolling transitions.